It is known, by the present assignee's own work, how to form and print microscopic 2-terminal vertical light emitting diodes (LEDs), with the proper orientation, on a conductive substrate and connect the LEDs in parallel to form a light sheet. Details of such printing of LEDs can be found in US application publication US 2012/0164796, entitled, Method of Manufacturing a Printable Composition of Liquid or Gel Suspension of Diodes, assigned to the present assignee and incorporated herein by reference.
FIG. 1 is a cross-sectional view of a layer of LEDs 16 that may be printed using the following process. Each LED 16 includes standard semiconductor GaN layers, including an n-layer, and active layer, and a p-layer.
An LED wafer, containing many thousands of vertical LEDs, is fabricated so that the bottom metal cathode electrode 18 for each LED 16 includes a reflective layer. The top metal anode electrode 20 for each LED 16 is small to allow almost all the LED light to escape the anode side. A carrier wafer, bonded to the “top” surface of the LED wafer by an adhesive layer, may be used to gain access to both sides of the LED for metallization. The LEDs 16 are then singulated, such as by etching trenches around each LED down to the adhesive layer and dissolving the exposed adhesive layer or by thinning the carrier wafer.
The microscopic LEDs are then uniformly infused in a solvent, including a viscosity-modifying polymer resin, to form an LED ink for printing, such as screen printing or flexographic printing.
If it is desired for the anode electrodes 20 to be oriented in a direction opposite to the substrate 22 after printing, the electrodes 20 are made tall so that the LEDs 16 are rotated in the solvent, by fluid pressure, as they settle on the substrate surface. The LEDs 16 rotate to an orientation of least resistance. Over 90% like orientation has been achieved.
In FIG. 1, a starting substrate 22 is provided. If the substrate 22 itself is not conductive, a reflective conductor layer 24 (e.g., aluminum) is deposited on the substrate 22 such as by printing. The substrate 22 may be thin and flexible.
The LEDs 16 are then printed on the conductor layer 24 such as by flexography, where a pattern on a rolling plate determines the deposition for a roll-to-roll process, or by screen printing with a suitable mesh to allow the LEDs to pass through and control the thickness of the layer. Because of the comparatively low concentration, the LEDs 16 will be printed as a monolayer and be fairly uniformly distributed over the conductor layer 24.
The solvent is then evaporated by heat using, for example, an infrared oven. After curing, the LEDs 16 remain attached to the underlying conductor layer 24 with a small amount of residual resin that was dissolved in the LED ink as a viscosity modifier. The adhesive properties of the resin and the decrease in volume of resin underneath the LEDs 16 during curing press the bottom LED electrode 18 against the underlying conductor 24, making ohmic contact with it.
A dielectric layer 26 is then printed over the surface to encapsulate the LEDs 16 and further secure them in position.
A top transparent conductor layer 28 is then printed over the dielectric layer 26 to electrically contact the electrodes 20 and is cured in an oven appropriate for the type of transparent conductor being used.
If needed to spread current, metal bus bars 30-33 are then printed along opposite edges of the conductor layers 24 and 28 and electrically terminate at anode and cathode leads (not shown), respectively, for energizing the LEDs 16. The bus bars 30-33 will ultimately be connected to a positive or negative driving voltage.
FIG. 2 is a top down view of FIG. 1. The cross-section of FIG. 2 is a horizontal bisection of FIG. 3. The locations of the LEDs 16 in the printed layer are random.
If a suitable voltage differential is applied to the anode and cathode leads, all the LEDs 16 with the proper orientation will be illuminated. FIG. 1 shows a light ray 38.
The above process is strictly for use with 2-terminal devices having a top electrode and a bottom electrode, since the locations of the LEDs on the substrate are random, and the LEDs can only be interconnected by sandwiching the LEDs between two conductive layers of any thickness. Further, the above process is strictly for forming an array of LEDs for generating light. The LEDs are not intended to perform any type of logic function since the array of parallel-connected LEDs simply forms a single diode.
It would be desirable to adapt the above-described printing/curing process to create complex printed electrical circuits involving 3-terminal transistors, diodes, and possibly additional types of components, to perform logic functions.